Generally, secondary batteries are classified into nickel-cadmium batteries, nickel-hydrogen batteries, lithium ion batteries, lithium ion polymer batteries and so on. Such secondary batteries are also classified into lithium-based batteries and nickel-hydrogen-based batteries. The lithium-based batteries are generally applied to small products such as digital cameras, P-DVD, MP3P, cellular phones, PDA, portable game devices, power tools and E-bikes, and the nickel-hydrogen-based batteries are generally applied to large products such as electric vehicles and hybrid vehicles, which require high power.
Meanwhile, an electric or hybrid vehicle should operate a motor requiring high power in order to move. For this purpose, a battery employed in the electric or hybrid vehicle uses as a power source an electricity output from a battery pack in which a plurality of battery cells are connected in series.
The plurality of battery cells included in the battery pack should keep their voltages uniformly in order to ensure stability, long life span and high power. An electric vehicle has a plurality of battery packs, each having a plurality of battery cells, and a battery management system (BMS) keeps each battery cell to a suitable voltage level while charging or discharging each battery cell of the battery pack. However, the plurality of battery cells may not be easily kept in an equivalent state due to various factors such as the change of internal impedance, so the BMS performs an operation for controlling the plurality of battery cells into an equivalent state.
In order to control the battery cells in a high voltage battery pack to have uniform voltages, a semiconductor switch was conventionally used to selectively connect a discharge resistor (or, a Buck resistor) to a battery cell whose voltage is higher than an average voltage, thereby exhausting the energy possessed by the battery cell and thus decreasing a voltage difference between battery cells.
In order to ensure safety while balancing the plurality of battery cells, if a voltage of a battery cell to be discharged is lowered to a certain voltage level, it is important to cut the connection between a discharge circuit and the corresponding battery cell. However, in some cases, the connection between the battery cell and the discharge circuit may not be cut at a proper time due to a failure of a control processor controlling the discharge of each battery cell or an error of a program algorithm. In this case, current is slightly discharged from the battery cell continuously, so the battery may come to an overdischarged state. If the battery cell is overdischarged, various dangers may occur. In particular, when coming to an overdischarged state, a lithium ion battery pack may cause serious problems, such as explosion.